1. Field of the Invention
The present invention relates to optical filters which are used in applications where spectroscopic measurements are used to determine the properties of substances such as chemicals and other substances.
2. Description of the Related Art
Optical filters are well known in applications involving spectroscopic measurement. Spectroscopic measurement is used to determine the properties and chemical composition of various substances in a sample based upon the optical characteristics of the sample. In a typical spectroscopic measurement, light (in the visible and non-visible range) is used to illuminate the sample over multiple frequency spectra. More than one optical frequency (wavelength) is used to more precisely determine the optical characteristics of the sample and also to subtract out interference. In some applications, the light reflected from the sample is detected, while in other applications light transmitted through the sample is detected to determine the optical characteristics of the sample. In addition, a combination of the transmission through the sample and the reflections from the filter may be employed.
The detected light is usually quantified to provide an indication of the "frequency response" of the sample at each of the frequency spectra. As is well known in the art, each substance has definable optical properties determined by the frequencies at which the substance reflects and absorbs light. Thus, the optical characteristics of a given substance may be quantified (e.g., plotted as intensity of reflected or transmitted light versus frequency) to provide an indication of the optical characteristics of that substance. Since different substances typically have distinct optical characteristics, quantified measurements of the optical properties of a sample containing several substances can serve as the basis for distinguishing among or making other measurements relating to the several substances within a sample. Precise measurements of the reflected or transmitted light can be used to determine the precise concentration of the various substances within a sample.
Some present spectroscopic measurement systems use multiple light emitting diodes (LEDs) or laser sources to provide light at the desired wavelengths. However, very expensive, high precision wavelength light sources must be employed in order to manufacture such a system with the necessary wavelength accuracy for each of the sources.
One alternative method of generating light at multiple frequencies involves rotating an optical filter between the sample to be measured and a broadband light source. Current optical spectroscopic devices, as identified by the inventor for use in the present invention, often require expensive custom-made filters which are used to generate a pattern of optical signals to be transmitted. One such filter, commonly known as a dichroic filter, comprises a rotating optically coated disk which includes regions of varying optical thickness. As the wheel spins, light from the broadband light source passes through different portions of the wheel so that light of various frequencies are passed by the filter to illuminate the sample. That is, the regions on the dichroic filter are formed in a pattern so that rotation of the optical disk results in the transmission of selected optical bands. In many previous applications involving precise spectroscopic measurement, optical filters have been designed with very high tolerances. Furthermore, the methods for manufacturing such filters have often precluded the possibility of manufacturing the filters by mass production. Thus, even optical filters of this kind may be prohibitively expensive to fabricate.